linux-kernel - Re: [PATCH] sched/cputime: Fix a mostly theoretical divide by zero

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Message-ID: <CAKfTPtBKm7WgwcUHcMXnH3PL3KFSniy02mb6Ag4eQ1fA3e+H3g@mail.gmail.com>
Date: Mon, 16 Jun 2025 14:51:56 +0200
From: Vincent Guittot <vincent.guittot@...aro.org>
To: "Li,Rongqing" <lirongqing@...du.com>
Cc: "mingo@...hat.com" <mingo@...hat.com>, "peterz@...radead.org" <peterz@...radead.org>, 
	"juri.lelli@...hat.com" <juri.lelli@...hat.com>, 
	"dietmar.eggemann@....com" <dietmar.eggemann@....com>, "rostedt@...dmis.org" <rostedt@...dmis.org>, 
	"bsegall@...gle.com" <bsegall@...gle.com>, "mgorman@...e.de" <mgorman@...e.de>, 
	"vschneid@...hat.com" <vschneid@...hat.com>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] sched/cputime: Fix a mostly theoretical divide by zero

On Mon, 16 Jun 2025 at 13:46, Li,Rongqing <lirongqing@...du.com> wrote:
>
> >
> > Sum of utime and stime can overflow to 0, when a process with many threads
> > run over total 2^64 ns

Theoretical is the right word; If all 2^32 possible threads belong to
the process, we can get an overflow to 0 after ~4sec run time of each
thread. But then how long will it take to have those 2^32 threads run
4sec on a system ...

It would be good to get number to show how realistic or not it could
be to reach this value

> >
> > Signed-off-by: Li RongQing <lirongqing@...du.com>
>
>
> Ping
>
> Thanks
>
> -Li
>
> > ---
> >  kernel/sched/cputime.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c index
> > 6dab4854..c35fc4c 100644
> > --- a/kernel/sched/cputime.c
> > +++ b/kernel/sched/cputime.c
> > @@ -579,7 +579,8 @@ void cputime_adjust(struct task_cputime *curr, struct
> > prev_cputime *prev,
> >               goto update;
> >       }
> >
> > -     stime = mul_u64_u64_div_u64(stime, rtime, stime + utime);
> > +     if (likely(stime + utime))
> > +             stime = mul_u64_u64_div_u64(stime, rtime, stime + utime);
> >       /*
> >        * Because mul_u64_u64_div_u64() can approximate on some
> >        * achitectures; enforce the constraint that: a*b/(b+c) <= a.
> > --
> > 2.9.4
>